Lithium secondary batteries have been used widely as power sources for portable electronic instruments, because they are small-sized and have high energy densities. As for their positive-electrode active materials, lamellar compounds, such as LiCoO2, have been employed mainly. However, these compounds have such a problematic issue that the oxygen is likely to be eliminated at around 150° C. under the fully-charged conditions so that this is likely to cause the oxidative exothermic reactions of nonaqueous electrolyte liquids.
Recently, as for positive-electrode active material, olivine-type phosphate compounds, Li“M”PO4 (e.g., LiMnPO4, LiFePO4, LiCoPO4, and the like), have been proposed. These compounds upgrade the thermal stabilities by means of using the divalent/trivalent oxidation-reduction reaction, instead of the trivalent/tetravalent oxidation-reduction in which an oxide like LiCoO2 serves as a positive-electrode active material; and have been attracting attention as compounds from which higher discharging voltages are available by means of further arranging the polyanions of hetero elements whose electronegativities are higher around the central metal.
However, in a positive-electrode material comprising an olivine-type phosphate compound, its theoretical capacity is limited to 170 mAh/g approximately because of the large molecular weight of phosphate polyanions. In addition, LiCoPO4 and LiNiPO4 have such a problem that no electrolytic liquids, which can withstand their charging voltages, are available because the operating voltages are too high.
Hence, as a cathode material that is inexpensive, which is more abundant in the amount of resource, which is lower in the environmental load, which has a higher theoretical charging/discharging capacity of lithium ion, and which does not release any oxygen at the time of high temperature, lithium-silicate-based materials, such as Li2FeSiO4 (with 331.3-mAh/g theoretical capacity) and Li2MnSiO4 (with 333.2-mAh/g theoretical capacity), have been attracting attention. These silicate-based materials are expected as a positive-electrode material for lithium secondary battery with much higher capacity, respectively. In addition, their discharging voltages are lower than those of phosphate-based ones by about 0.6V approximately, which is a reflection of the fact that the electronegativity of Si, a hetero element, is smaller than that of P. Thus, there is such a possibility that Co and Ni are employable as a doping element to the silicates.
Of these silicate materials, Li2FeSiO4 is a material showing the highest charging/discharging characteristic ever that has been reported at present, and exhibits a capacity of 160 mAh/g approximately. However, Li2FeSiO4 has not yet arrived at obtaining a charging/discharging characteristic that goes beyond 169.9 mAh/g, the theoretical capacity of LiFePO4 that is one of the current materials.
As for synthesizing methods for the silicate-based compounds being mentioned above, the hydrothermal synthesis method, and the solid-phase reaction method have been known.
Of these methods, it is feasible to obtain fine particles with particle diameters of from 1 to 10 nm approximately by means of the hydrothermal synthesis method. However, in silicate-based compounds being obtained by means of the hydrothermal synthesis method, there are the following problems: doping elements are less likely to dissolve; the phases of impurities are likely to be present mixedly; and additionally battery characteristics being expressed are not quite satisfactory.
On the other hand, in the solid-phase reaction method, although it is feasible to dissolve doping elements because it is needed to cause reactions at such high temperatures as 650° C. or more for a long period of time, the resulting crystal grains become larger to 10 μm or more, thereby leading to such a problem that the diffusion of ions is slow. Besides, since the reactions are caused at the high temperatures, the doping elements, which have not dissolved completely, precipitate to generate impurities in the cooling process, and so there is also such a problem that the resultant resistance becomes higher. In addition, since lithium-deficient or oxygen-deficient silicate-based compounds have been made due to the heating being done up to the high temperatures, there is also such a problem that it is difficult to increase capacities or to upgrade cyclabilities (refer to following Patent Literature Nos. 1 through 4).